[GHC] #11732: Deriving Generic1 interacts poorly with TypeInType

[GHC]

#11732: Deriving Generic1 interacts poorly with TypeInType
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        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 No instantiation check. But what GHC does right now is, for every class in
 a datatype's `deriving` clause, drop as many type parameters as there are
 arguments to the class being derived. For example

 {{{
 data Foo a deriving (Show, Functor)
 }}}

 GHC knows that we mean `instance Show (Foo a)` and `instance Functor Foo`.

 But

 {{{
 data Proxy k (a :: k) deriving Functor
 }}}

 would try `instance Functor (Proxy k)`, which is ill-kinded. GHC is surely
 clever enough to figure out that it should really do `instance Functor
 (Proxy *)`, but I think it should refrain from doing this.

 More to the point of this ticket: I have a bad feeling my fix for #11357
 is utterly broken, in that it uses the `tc_args` from the instance tycon
 despite using visibilities from the parent tycon. Of course, the `tc_args`
 don't match up. I suppose a `mkFamilyTyConApp` would work nicely here.

 But you seem to know much more about generics in GHC than I do. Do you
 think you could make this fix? I really do think we just need to use
 `mkFamilyTyConApp` in the right spot here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11732#comment:4>
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